Combine oil, lime juice, vinegar, jalapeno, cilantro, salt, chili powder, and black pepper in a medium-sized bowl. Add tuna and coat well. Marinate for 1 to 2 hours.
Heat a skillet over medium-high heat. Add small amounts of tuna at a time, turning every 10 to 15 seconds until lightly browned, 2 to 3 minutes per side.